We studied the effect of aging on the repair of a vector containing the chloramphenicol acetyl transferase gene. This methodology more closely approximates in vivo DNA repair than other methods in that only repair of transcribed segments is measured. Techniques employing these shuttle vectors have been proved to be accurate and reproducible in several studies. No differences in repair were found in studies of primary cultured rat embryo fibroblasts from young and old donors. In addition the repair capacity of rat embryo fibroblasts did not display any change with passage level. A study of repair in human fibroblasts from donors of various ages has begun using the shuttle vector system.